Custom backends may rely on networking or other unreliable implementations to support revsets, this change allows them to return errors cleanly instead of panicking.
For simplicity, only the public-facing Revset and RevsetGraph types are changed in this commit; the internal revset engine remains mostly unchanged and error-free since it cannot generally produce errors.
Stacking at AliasExpanded node looks wonky. If we migrate error handling to
Diagnostics API, it might make sense to remove AliasExpanded node and add
node.aliases: vec![(id, span), ..] field instead.
Some closure arguments are inlined in order to help type inference.
It was convenient that expression nodes can be compared in tests, but no
equivalence property is needed at runtime. Let's remove Eq/PartialEq to
simplify the extension support.
This will allows us to parse "file(..)" arguments as fileset expression by
transforming AST for example. I'm not sure if that's good or bad, but we'll
probably want to embed fileset expressions without quoting.
parse_expression_rule() is split to the first str->ExpressionNode stage and
the second ExpressionNode->RevsetExpression stage. The latter is called
"resolve_*()" in fileset, but we have another "symbol" resolution stage in
revset. So I choose "lower_*()" instead.
If we ever implement some sort of ABI for dynamic extension loading, we'll need these underlying APIs to support multiple extensions, so we might as well do that first.
These .wrap_<type>() functions aren't supposed to capture resources from the
language instance. It was convenient that wrap_() could be called without fully
spelling the language type, but doing that would introduce lifetime issue in
later patches.
I added type alias L to several places because the language type is usually
called L in generic code.
